The verb 'santify' denotes, especially in the Old Testament, the human act of setting apart an object for God's sole use, in which case the object is said to be 'holy'. To sanctify oneself is to take such special acts as to denote that the person is set apart for God, whether in a life of service or of special devotion. Sanctification, however, is also a process of spiritual growth expected of all Christian believers; Louis Berkhof defines sanctification as "that gracious and continuous operation of the Holy Spirit, by which He delivers the justified sinner from the pollution of sin, renews his whole nature in the image of God, and enables him to perform good works."
Some theologians, for the sake of greater rigour in thinking through the subject, distinguish between different phases of sanctification. First comes positional sanctification, which is defined as the one-time act by the Holy Spirit at the moment of our conversion when we are positionally set apart for God and authorized to live henceforth as His child. Next comes progressive sanctification, the life-long process of battling with and overcoming the pollution, the practice, and the power of sin in our lives. Final sanctification comes when we die and no longer have to deal with the groans of our mortal flesh.
Some Christians, perhaps best represented by the Methodists in general, believe that this process of spritual growth towards holiness can be made so completed in a believer's lifetime as to render a Christian perfect in holiness. This doctrine is given the name 'entire sanctification.' While the hope of such perfect holiness is held out before us in Scriptures—it may even be said that Jesus encouraged such a life goal when he commands "Be perfect, therefore, as your heavenly Father is perfect" (Matt 5:48)—it is doubtful if it has ever been accomplished in any person other than our Lord Jesus. If there was ever such a person, its truth would have been impossible to verify, at any rate. Such a person himself would never have dared claimed it to be so, and others have no way of ascertaining the inner thoughts and motivations of his heart to be able to say so. Nevertheless, perfect holiness is a goal towards which all believers should strive because it pleases God and makes Him pleased to dwell and to deal with us, and simply because Jesus commands it.
Sanctification and holiness is the forgotten gift in the modern church. It is seldom preached about or taught in local churches and—as evident from the paucity of resources available—a subject visibly missing as worthy of consideration in theological circles. As lay-preachers you can make a difference. Rediscover this precious gift from God, live it, and teach it.
You may also wish to read the entries on Holiness, Clean-Unclean 
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Further Reading & Resources:
☰Louis Berkhof, Systematice Theology. Alberith Edition. html-aar If you are new to the subject, start with this before you venture into the resources below.
☰ J. C. Ryle, Holiness. Its Nature, Hindrances, Difficulties, and Roots. Alberith Edition.
html-aar. This is only partially complete.
Arnett, William M., Entire Sanctification. Papers. 12. Lexington, KY: First Fruits Press, 2013. html N
Christian Wismer Ruth, Entire Sanctification Explained. Wesleyan Heritage Publications, 1998. pdf N
☰ Richard L. Mayhue, "Sanctification: The Biblical Basics," McMaster Seminary Journal 21/2 (Fall 2010): 143-157. Pdf N
Arthur W. Pink, The Doctrine of Sanctification (Pensacola, FL: Chapel Library, 2013). pdf N
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